524 DEVELOPMENT OF THE SIMPLE TISSUES, BY S. STEICKER. 



laminro ; the lower the mucous and intestinal fibrous lamina. The two 

 upper layers he designated the animal ; the two lower the vegetative. 



Reichert* gave another account. He distinguished in the blasto- 

 derm of the fecundated, but non-incubated ovum a layer of corpuscles, 

 from which a membrane (the investing membrane) was developed 

 during the first hours of incubation. The formation of this tunic 

 Reichert regarded as being the first condition for the further develop- 

 ment of the embryo, since the yolk cells assumed a laminated arrange- 

 ment upon its inner surface. The rudiment of the nervous system, he 

 maintained, first makes its appearance, then that of the middle lamina, 

 which, on account of its position between the upper and lower germ 

 layers, he termed the membrana intermedia. After the completion of 

 the formation of this membrane the development of the lower layer 

 commences, the yolk cells depositing themselves upon the lower sur- 

 face of the former at a time when the embryo begins to be constricted 

 off from the blastoderm. As regards the significance of the blastodermic 

 layers in the further development of the embryo, he considered that 

 the upper lamina takes no part in the formation of the embryo, and that 

 it disappears during the embryonic life of the organism. In regard 

 to the middle lamina, he was the first to show that a division of the 

 lateral plates occurs, and. that the middle lamina participates in the 

 formation of the walls of the body. He maintained also that the 

 corneal layer of the integument, the glands of the skin, the muscular, 

 osseous, and vascular systems, as well as the intestinal fibrous layer, 

 with the associated glandular organs, are developed from this lamina. 

 The lower lamina he regarded as representing simply the rudiment 

 from which the epithelium of the digestive organs is developed. 



Remak first noticed that the blastoderm of the fertilized but non- 

 incubated egg is composed of two layers. The next changes were 

 referable, according to him, to the inferior layer, which becomes thicker, 

 though it is always looser and less transparent than the upper one. 

 Then follows an histological differentiation of its elements, a layer of 

 cells becoming detached, and covering the inferior surface like an 

 epithelium. As regards the relation of the germ layers to each other 

 in the area of the germinal area (Fruchthof), the upper and middle 

 appear to become thicker, and coalesce at a very early period in the 

 centre of this area. The inferior layer, however, takes no part in this 

 coalescence. With the exception of the middle part of the germinal 

 area, all the three layers may be easily separated from each other 

 throughout their whole extent, both in the vascular area, and in the 



* Loc. tit. 



